In its broad aspects, the present invention is directed to secondary electrical energy storage systems of the aqueous type which utilize a metal electrode such as zinc which in combination with a reducing agent undergoes an electrochemical reaction in a cell to produce an electrical current. A metal halogen hydrate electrical energy storage system of the type to which the present invention is applicable is fully described in U.S. Pat. No. 3,713,888, entitled "Halogen Hydrates", issued Jan. 30, 1973. This patent is owned by the same assignee as the present invention and details thereof beyond those herein described are incorporated in this application by reference. Metal halogen hydrte electrical energy storage systems or secondary storage batteries are conveninetly categorized as being of the high energy density (H.E.D.) type because of their capacity to supply upwards of 50 watt hours of electric power per pound of weight. This high electrical energy capacity coupled with the compactness and low weight of such secondary storage batteries has rendered them particularly satisfactory for use as principal and auxiliary sources of electrical energy in either mobile (electric vehicles) or stationary (utility load leveling) power plant systems.
The present invention pertains primarily to zinc halogen battery systems, and more prticularly to zinc chlorine battery systems, although it should be appreciated that the invention described herein may be equally applicable to other metal halogen battery systems. The chemical reactions which occur in a zinc chlorine hydrate battery are relatively straightforward. During charge, the electrolyte (a solution of zinc chloride in water) is flowed through the battery with the aid of a circulator. As electrical direct current is passed through the battery from an external source, zinc metal is electro-deposited on the zinc electrode (typically relatively dense graphite) of the battery as a uniform, non-porous solid. Simultaneously, chlorine gas, generated at the chlorine electrode (typically porous graphite or ruthenia-catalyzed porous titanium) is carried away with the circulating electrolyte stream. Outside of the battery, the mixture of chlorine gas and electrolyte is cooled and a pale yellow solid called chlorine hydrate is formed. The solid chlorine hydrate (Cl.sub.2.xH.sub.2 O) is retained separate from the battery, and the remaining electrolyte is returned.
During discharge, the aqueous zinc chloride electrolyte is again circulated through the battery thereby carrying chlorine, which is slightly soluble in the electrolyte, to the chlorine electrode of the battery. Zinc metal reacts at the zinc electrode to form zinc ions, and chlorine reacts at the chlorine electrode to form chloride ions. As the battery is discharged, chlorine is returned to the electrolyte from the storage area by controlled heating of the chlorine hydrate. These processes continue until the chlorine-hydrate in the store is dissipated and the battery is discharged.
For some applications, such as vehicular use, the discharge is often a partial discharge, which poses special problems as compared to a complete discharge. Test results on zinc chlorine batteries, and more particularly on the zinc electrode in such batteries have, in general, shown unacceptable zinc-on-zinc recyclability. It should be recalled that the zinc electrode, typically made from relatively dense graphite, provides a surface for the electron transfer reactions in charge and discharge between metallic zinc and zinc ions. Zinc-on-zinc recyclability refers to the ability of the negative battery electrode (the zinc electrode) to accept a recharge after partial discharge, which in effect means the ability of the electrode to accept a fresh zinc plate on top of an older zinc plate, the older plate having been partially dissolved in the preceding discharge. In the case of partial discharge, zinc is oxidized off the electrode, but very often in a somewhat irregular manner, thus leaving a poor surface layer for subsequent deposition or replating of zinc during charge. As a result, the newly deposited zinc forms a dendritic or nodular structure, which may lead to internal short-circuiting of the battery.
One solution which has been proposed to the problem of dendrite formation is the use of additives in the electrolyte solution; however, this additive approach has not met with total success. Organic additives such as those used in typical zinc plating baths, are usually oxidized or decomposed by the oxidizing agents in most rechargeable batteries. Also, by various mechanisms they may interfere with the reversibility of either electrode. It has also been found that some additives tend to precipitate or salt out during repeated recharging; examples of such additives being benzotriazole, benzene sulfonamide, toluene sulfonamide, chlorotoluene sulfonamide and thiourea. Lastly, attempts at using certain organic additives, such as those described in U.S. Pat. No. 3,793,079 and U.S. Pat. No. 3,811,946 both owned by the assignee of the present invention, although effective, have not met with total success in all cases, since they sometimes tend to degrade over extended periods.
The chemical instability of many organic additives in metal halogen battery systems may in some circumstances be circumvented by using stable inorganic additives. One such inorganic additive, thallium, while providing some leveling for the initial charging process, is generally unacceptable for cycling zinc on zinc due to morphological changes encountered on partial discharges. This is due in part to the fact that the additive, which codeposits with zinc during the charge so as to be distributed throughout the plate, is held at the electrode surface on discharge, when the zinc is oxidized, by virtue of the highly reducing conditions existing at the electrode surface. The remaining surface layer provides poor deposition sites for the replating of zinc, thereby causing the newly deposited zinc to form a dendritic or nodular structure. In the past, when good zinc-on-zinc recyclability was desired, zinc chloride without such an additive would be used and cell conditions would be carefully chosen to achieve the desired capacity and recyclability. However, the absence of an additive results in a granular zinc deposit making the system somewhat less forgiving in respect to variations in contaminant levels and operating conditions. On this basis, an additive that has no adverse effect on recyclability would be desirable.
The improved electrolyte composition comprising the present invention overcomes the foregoing problems by enabling a recharging of zinc halogen batteries at a rate practical for normal use and wherein the metal is redeposited during the recharging process in the form of a substantially smooth, dense, and adherent metallic deposit. Thus the partial discharge-recharge characteristics of the battery are substantially improved.